Organic carbon and microbial biomass in two soil development chronosequences following glacial retreat

Retreating glaciers successively expose mineral substrate which is colonised within a few years by soil biota, lichens and plants. Two such successions which cover the first five decades after exposure were studied in the Swiss Alps. Sites differed in parent material and precipitation. At each site, five strata of different age were dated by distance from the current glacier gate and records of annual retreat. The proportion of fine material (< 63 mu m) differed by a factor of 2 between sites and a factor of 3 within sites. From the youngest to the oldest stratum, plant cover increased from < 5 to > 85% and organic carbon content (C-org) in the bulk material increased by 6 times at the wetter site and by 16 times at the drier site. Even so, partitioning of C-org between particle size fractions 2000 to 63 mu m and < 63 mu m was the same at both sites (6 : 4), and so was the proportion of microbial C (C-mic) in C-org (5%). Coefficients of variation across all 10 strata studied were only 22% for C-org partitioning and 21% for the proportion of C-mic in C-org. Soil carbon models frequently apply prescribed partitioning factors in simulating organic matter input into soil. Our findings lend support to such an approach.